The present invention relates to an image pickup apparatus of the type having a rotary shutter which controls the amount of time the image forming light strikes an image element.
Generally, electronic still picture cameras using a still video floppy disk as an image recording medium have in their image pickup system a solid state image sensor consisting of a large number of small photoelectric conversion elements arranged in a matrix, for example devices known as CCD arrays, MOS arrays and the like. The use of such solid state image sensors contributes to the low cost of manufacturing electronic still picture cameras and the compactness of cameras for convenience of carrying.
The above-mentioned solid state image sensor is driven by scanning signals to transfer to a shift register image information in the form of signal charges from the photoelectric conversion elements in the arranged order. When the signal charges are transferred to the shift register from all the photoelectric conversion elements, an operation for taking a single still picture is completed. This operation can be successively repeated by successively driving the solid state image sensor. However, because the electronic still camera generally cooperates with shutter means, it is usual to use, as image information, photoelectric output signals retrieved from the image sensor in a time period defined by the shutter means, for example approximately 1/60 to 1/1000 sec., in order to record one frame of still picture image.
It has been attempted to use as a shutter incorporated in the above-described electronic still camera a rotary shutter which is adapted to rotate always in the path of image forming light.
The rotary shutter is driven by means of a servodriving system to rotate at a constant speed in synchronism with scanning the image sensor. That is to say, the rotary shutter, after having reached a constant speed of rotation, begins to open simultaneously with every vertical synchronizing signal supplied to the image sensor. Consequently, it suffices to start an image pickup operation immediately in response to a first vertical synchronizing supplied to the image sensor after the operation of shutter initiating means. In other words, it is unnecessary to synchronize opening the rotary shutter with the vertical synchronizing signal every image pickup operation in order to achieve a favorable operation. In addition, the provision of the rotary shutter adapted to continuously rotate contributes to a drastic reduction of time intervals at which consecutive still images are recorded.
One of the problems with which the use of the rotary shutter is confronted is that the synchronized rotation of the rotary shutter is not immediate at its beginning, because of difference of starting characteristic between a motor for the rotary shutter and a pulse generator which provides the vertical synchronizing signals for scanning. When the rotary shutter rotates under asynchronized conditions, the rotary shutter often either fully or partly blocks the image sensor which is being scanned. Therefore, when an image pickup operation is conducted under such asynchronized conditions, no image information signal or an insufficient image information signal is retrieved.
In the case of the electronic still picture cameras of the type having an electronic viewfinder for displaying images on a CRT or LC screen, when the rotary shutter is in an asynchronized condition, there appears an undesirable image of the whole or a part of the rotary shutter on the finder screen. Such undesirable or confusing image mars the otherwise perfect viewfinder image and often misinforms the camera operator that the camera is broken.
Another problem is the current consumption of batteries. Before the rotary shutter reaches a constant speed of rotation, if insufficient image information signals are retrieved, undesirable images appear on the screen of the electronic viewfinder. It is therefore undesirable to supply power to circuit units of an image recording system and an electronic viewfinder and to keep them active.